This invention relates to a method of prestressing at least one of the glass sheets of a sandwich safety glass, said sheets being bent in a bending furnace by the action of gravity, in superimposed relationship on a bending die having a substantially horizontal shaping surface, the die together with the glass sheets in superimposed relationship thereon passing through the furnace and the superimposed glass sheets, after passing through the bending zone in which the sheets are heated to the softening temperature and bent, being taken past a blow system which subjects the bent sheet or sheets to the action of cooling air and which is disposed in the furnace in a temperature zone above the transformation point but below the deformation point of the glass, and apparatus for prestressing at least one of the superimposed sheets of a sandwich safety glass, which are bent simultaneously by the action of gravity in the bending furnace, comprising a substantially horizontal die which travels through the furnace in a direction parallel to the axis of curvature of the sandwich, and which supports the superimposed sheets, a bending furnace zone through which the die travels and in which the superimposed sheets are heated to the softening temperature and bent, and a cooling zone, which follows the bending zone and is disposed in a temperature zone above the transformation point but below the deformation point of the glass, said cooling zone comprising a blow system which subjects the bent sheet or sheets to cooling air, suitable more particularly for performing the above-described process.
In a known process and a known apparatus of the type described hereinbefore (DOS No. 26 50 206), the two superimposed sheets are bent at a temperature above the transformation point and then vertically subjected to cooling air so that only the edge of the sheet is covered by the streams of cooling air, so that compression stresses are produced there. The compression stresses produced at the edge of the sheet with the process and apparatus according to DOS No. 26 40 206 are intended to guarantee clear vision in the stress-free central zone in the event of breakage of the sandwich type safety glass, which is of use more particularly as a windscreen for motor vehicles. However, it has been found, particularly with sandwich type safety glass consisting of individual sheets of different thicknesses, such as are frequently used as motor vehicle windscreens, in which the thinner sheet of the sandwich is disposed on the inside of the motor vehicle, that the safety glass does not have the required resistance to breakage if the central zone, particularly of the thin inner sheet, is not prestressed or toughened but is substantially stress-free, and in addition it does not give a breakage pattern in the form of extensive cracking which is desirable as a compromise between the glass strength requirements, on the one hand, and low risk of injury due to cuts, on the other hand.
DAS No. 10 35 328 discloses a process in which the entire surface of the individual sheets is subjected to cooling air in the bending furnace, so that they are prestressed. However, this process is not suitable for satisfactorily improving the properties of the individual sheets of a safety glass sandwich bent in superimposed relationship on the bending die to give slight prestressing. German patent specification No. 616 445 also describes the prestressing of bent sheets by cooling air nozzles, although here again there is no guarantee of satisfactory slight prestressing of the individual sheets of the sandwich bent in superimposed relationship on a bending die. Other prestressing systems operating with cooling air are described, for example, in DAS No. 10 80 742, German patent specification No. 11 14 993 and DAS No. 20 00 271, but all the processes and apparatus described therein are not specifically suitable for slightly prestressing or toughening at least one of the individual sheets of a safety glass sandwich are bent in superimposed relationship on a bending die.
The object of the present invention is to provide a process and apparatus of the type indicated hereinbefore whereby at least one of the individual sheets of a safety glass sandwich, bent in superimposed relationship on a bending die by gravity, is slightly prestressed. The object of the process and apparatus is to give the slightly prestressed and particularly very thin sheet of the sandwich the properties of high-strength glass.